Presented, and published in the proceedings from the
12th
International Conference on Low Frequency
Noise and Vibration and its Control held in Bristol, UK,
18th to 20th September 2006. Also Presented at the 2001, 142nd annual Acoustical Society
of America, American Institute of Physics, International Conferenceayman's version published in Cat Watch Newsletter
from the Cornell University College of Veterinary Medicine, Healthy Pet
Magazine, Cats, Cat Fancy, and others

**Notice to
readers. This paper is not designed to give
veterinary or medical advice.**

Briefly
what we did......Forty-seven felids were recorded including cheetahs, ocelots, pumas, domestic cats, and servals. A Sony TCD-D8 Digital Audio Recorder (DAT) and Statham Radio microphones recorded the purrs.
FFTs and spectrographs were performed using National Instrument's Polynesia. An accelerometer was also used to measure domestic cat purrs.
Then we analyzed hours and hours and hours of purrs! This is what we
concluded...

All smaller felids,
including the domestic cat, caracal, serval, puma, ocelot, and even some large
cats such as lions and cheetah purr. Since the 1970's no one has pursued
research into the 3000 year old question, "Why do cats purr?" Perhaps it is
because, one, we didn't have the knowledge we have now, and two, it was simply
easier to assume that cats purr when they are content, which cannot be
argued-they do purr when they are content. However they also purr when
frightened, severly injured, giving birth and even while dying. Because of this,
the contentment hypothesis clearly cannot be the only reason cats purr:.

(1) A vocalization is used to
display a particular emotion or physiological state. This enables an individual
in society or pack to be able to express themselves. As any cat owner knows
well, there are different "meows" for different emotions. A cat owner knows the
difference between their cat's "fearful hiss", "food meow",
and "let me oooouuutttt!!!" This cannot be
applied to the purr however. Cats purr even when they give birth and when
severely injured in a barren cage at the veterinarian's. There are cases of cats
purring when they are in grave physiological or psychological stress, as well as
when they sit on your lap. Therefore, purring really cannot be considered a
vocalization, as the purr is produced under differing emotions or physiological
states. As an example, a cat hissing when he/she was happy and when he/she was
scared, would confuse the rest of the cat's companions and probably would lead
to him/her being ostracized.

(2) Natural selection insures
that a particular trait be advantageous to an animal. Admittedly, there is some
benefit obtained from purring to ones' self or to kittens, (a sort of
kitty lullaby if you wish). Yet, there does not appear to be a strong
'survival' advantage to this behavior, unless, of course, you wish to constantly
display submission. For the purr to exist in different cat species over time,
geographical isolation etc. there would likely have to be something very important (survival mechanism)
about the purr. There is also would have to be a very good reason for energy expenditure (in this
case creation of the purr), when one is physically stressed or ill. The
vibration of the cat's diaphragm, which with the larynx, creates the purr,
requires energy. If an animal is injured they would not use this energy unless
it was beneficial to their survival. When was the last time you heard
someone singing, or humming to themselves (before pain drugs, and they weren't
on any when they came in) when they were in the emergency room with a
broken leg? The purr has to be somehow involved with survival.

Old wives' tales usually
have a grain of truth behind them, and most people have heard of a cat's "nine
lives." There is also an old veterinary adage still repeated in veterinary
schools which states, "If you put a cat and a bunch of broken bones in the same
room, the bones will heal." Any veterinary orthopedic surgeon will tell you how
relatively easy it is to mend broken cat bones compared with dog bones which
take much more effort to fix, and take longer to heal. There is excellent
documentation of the cats' quick recovery from such things as high-rise
syndrome. First mentioned by Dr. Gordon Robinson in 1976, high-rise syndrome
was later studied by Whitney, W., and Mehlhaff, C., (1987) the Journal of the
American Veterinary Medical Association. They documented 132 cases of cats
plummeting many stories from high rise apartments, (average 5.5 stories)
some suffering severe injuries. Interestingly, 90% of these cats survived. The
record for survival from heights is 45 stories, however most cats suffer from
falls of 7 stories or more and manage to live.

There has been some research which that suggests
that domestic cats are in general less prone to postoperative complications
following elective surgeries. Using computer records, Pollari and Bennet, (1996)
state that complications following surgery for dogs undergoing castration to be
averaged at 9.8%. The same surgery for cats lists the rate of complications to
be 1.2%. Dogs undergoing overiohysterectomies (OHE) had complications 17.4% of
the time and cats 8.4%. In another study by the same authors comparing paper
records with computerized documentation, dogs undergoing castration
complications varied from 2.4% to 22%, in cats 0.0% to 6.3%. With OHE
complications varied from 6.5% to 17.7% in dogs and 3.6% to 16.% in cats. Lund
et al. (1999) the records of 31,484 dogs and 15,226 cats at 52 veterinary
practices to determine the most common disorders. Arthritis in dogs was listed
as 2.4% of the population and was not listed as being reported in the cat. The
prevalence of lameness in dogs occurred 3.1% of the time, in cats it is not
mentioned as being reported. Healthy dogs were listed as 6.8% of the dog
population, healthy cats 9.5%.

Bone and
muscles/ligaments

Although it is
impossible to standardize the healing time for dogs and cats in clinically
occurring fractures, due to the type of fracture, amount of trauma to soft
tissues, the type of treatment, the standard evaluation time or the after care,
some general statements can be made, (Johnson, 2001). Cats do not have near the
prevalence of orthopedic disease or ligament and muscle traumas as dogs do.
Additionally, Toombs et al. (1985) suggests that non-union of fractures in cats
is rare.

Osteo diseases that
are rarely found in cats but can be found in all breeds and sexes of dogs
include; Osteochondritis dissecans of the proximal humerous, scapulohumeral
joint luxations, hip dysplasia. Osteo diseases in which cats are completely
unaffected include fragmented coronoid process, ununited anconeal preoceese,
traumatic elbow luxation, elbow subluxation, and legg-perenes. Osteosarcoma
occurs much less frequently in the cat then in the dog. Johnson, 1999.
Osteoarthritis and CPPd have only been found in large cats that were raised in
zoological parks. The frequency of effected cats in the wild is apparently so
low, that they are infrequently effected by these diseases in the wild.
(Rothschild et al., 1998)

Myeloma is a tumor
of plasma cells originating in the bone marrow. Only eight cats with multiple
myeloma have been reported to have osteolytic bone lesions. 56% of all dogs
reported with this condition involve bone. The metastatic behavioral differences
between dogs and cats is that tumors in the dog involve the whole body, whereas
in the cat it involves the distal ends of the extremities.In Lameness

With regard to the
prevalence of ligament and muscle injuries and disease, those that are seen
regularly in dogs but not in cats include, cranial crutiate ligament ruptures,
meniscal injuries (torn ligaments), muscle contusions and strains, muscle
contracture and fibrose, quadricepts contractor and inialsinatus, bicipital
tenosynovitis, medial patellar luxation, lateral patellar luxation,
osteochronditis dissecans of the stifle, and ligamentous injury of the tarsus.
Johnson

One explanation for
the lack of trauma or disease found in cat bone and muscle/ligaments is that
cats are more sedentary then dogs, however this is a supposition and is not
documented.

Respiratory

There have been
studies that indicate that purring can aid in dyspnea as Cook in 1972 suggests.
Kidd et al. in 2000 found in a study with 11 cats and 17 dogs with acute and
subacute myocardial necrosis, none of the cats in the study had dyspnea,
although all the dogs did. The overall incidence of primary lung tumors in the
dog is 1.24%, and in the cat, .38% (Miles, 1988)

Tissue

Free skin grafting
is often used for the treatment of large skin defects on the distal limbs of
dogs and cats. However while using this technique in dogs, the overlapped skin
edges of the graft usually become necrotic by 3 days postoperatively, and need
to be debrided. In cats, the grafts are usually viable even after six days.

Unfortunately,
there has been no research that has attempted to explain the
extraordinary ability cats have for healing themselves.

Just two years
ago, Dr. Clinton Rubin and his associates made a fantastic discovery. They
found that exposure to frequencies between 20-50 Hz (at low dB) creates the
robust striations of increased bone density, Clinton Rubin, (1999), Strain
mediated augmentation of bone mass and morphology: Is it possible to harness the
anabolic potential of mechanical stimuli without necessarily requiring exercise?, Wellcome
Trust. In one study chickens were placed on a vibrating plate every day for 20
minutes, and grew stronger bone, National Geographic, January 2001, p.
11. This discovery of anabolic frequencies between 20- 50 Hz (at low dB), is a
tremendous breakthrough. Astronauts in space loose bone density in zero gravity,
and this method could help them maintain healthy bones. Dr. Rubin's group has
begun research trials with humans, designed to test whether this non-invasive
method halts osteoporosis and perhaps even renews bone growth in post-metapausal
women; J. Zhi, and M. Hadjrargyrou, (1999) The expression of a novel and a
known gene, unregulated by disuse is down regulated by anabolic mechanical
stimulation, American Society of Bone and Mineral Research. This method is
not yet FDA approved, although it is hoped it will be soon. Additionally, Chen
et.al (1994) The effects of frequency of mechanical vibration on
experimental fracture healing, Zhonghua Wai Ke Za Zhi, in his work with
rabbits, found that frequencies of 25 and 50 hertz promote bone strength by 20%,
and stimulate both the healing of fractures, and the speed at which the
fractures heal.

There is also
documentation that low frequencies, at low dB are helpful with regard to pain
relief, and the healing of tendons and muscles. Vibrational stimulation between
50-150 Hz has been found to relieve suffering in 82% of persons suffering from
acute and chronic pain (Lundeberg, 1983). In 1999, M. Falempin and S.F.
In-Albon discovered that mechanical vibration at 120 Hz counteracted atrophy in
tendons after hind-limb muscle loading. Biomechanical stimulation which uses
mechanical vibration of standardized frequencies from 18 - 35 Hz is used in
Russian sports medicine. This technique improves the relaxation of strained
muscle structures and increases the stretching ability of capsules and tendons.
Lake in 1992, found that biomechanical stimulation prevents a decrease in muscle
strength and muscle mass and the oxidative capacity of thigh muscles, following
knee immobilization after sports injuries. The use of low frequency therapy also
applies to tendon healing. It can increase the mobility of upper ankle joints
by 16- 19 %, Klysczt et. al, 1997, Biomechanical stimulation therapy as
physical treatment of arthrogenic venous insufficiency, Hautarzt. Exposure
to frequencies between 2-100 Hz results in in the reduction of muscle spasms and
more pronounced reduction of the spasms occurs the longer the treatment is
applied, (D. Ardic, A. Buljina, 2000). After ten days of short periods of
biomechanical stimulation, upper mobility of ankle joints improved by 16 and 19
degrees and was accompanied by the healing of venous ulcerations after skin flap
transplantation, (Klysch, T. et al., 1997). It is interesting to note that
Biomechanical stimulation is also used in public gyms and work-out centers to
increase muscle mass. A web search will bring up many manufactures of such
equipment.

It has also
been found that in- phase chest wall vibration at 100 Hz, is known to decrease
dysponea in patients with chronic obstructive pulmonary disease while at rest
(Cristiano and Schwartzstein 1997; Nakayama, et al., 1998; Sibuya, 1994).

In Summery:
Vibrations between 20-140 Hz are therapeutic for bone growth/fracture healing,
pain relief/swelling reduction, wound healing, muscle growth and repair/tendon
repair, mobility of joints and the relief of dyspnea.

We think that this
research could help explain why cats purr, and here is why:

Fauna
Communications has recorded many cats' purrs, at a non-profit facility and the
Cincinnati Zoo , including the cheetah, puma, serval, ocelot and the
domestic house cat. After analysis of the data, we discovered that cat purrs
create frequencies that fall directly in the range that is anabolic for bone
growth.

·The dominant and fundamental frequency for three species of cats'
purrs is exactly 25 Hz, or 50 Hz the best frequencies for bone growth and
fracture healing. All of the cats purrs all fall well within the 20 - 50 Hz
anabolic range, and extend up to 140 Hz.. All the cats, except the cheetah have
a dominant or strong harmonic at 50 Hz.

·The harmonics of three cat species fall exactly on or within 2
points of 120 Hz which has been found to repair tendons. One species within 3 Hz
and one within 7 Hz.

·Eighteen to thirty-five Hz is used in therapeutic biomechanical
stimulation for joint mobility. Considering the small size of many of these
cats, especially the domestic cats, it is interesting to note that that all of
the individual cats, have dominant frequencies within this range. In fact, some
of the cats, have 2-3 harmonics in this range.

·The frequencies for therapeutic pain relief are from 50-150 Hz. All
of the individual cats have al least 5 sets of strong harmonics in this range.

·Therapeutic frequencies for the generation of muscle strength lie
between 2-100 Hz. All of the individual cats have al least 4 sets of strong
harmonics in this range.

·Therapy for COPD uses 100 Hz, all of the individual cats have a
dominant frequerncy of exactly 100 Hz.

There is
another clue found in a study performed by Dr. T. F. Cook, (1973) The relief
of dyspnoea in cats by purring, New Zealand Veterinary Journal. A dying cat
who could not breath (they were considering euthanasia), was found to breath
normally once it began purring. The purring opened up the cat's airway, and
improvement was "remarkable and the next day commenced to eat...." Three
species of cats have a strong harmonic at exactly 100 Hz, the vibrational
frequency found to relieve dyspnea. One species within 2 Hz and one species
within 7 Hz of 100 Hz. It could be that the cat's purr decreases the
breathlessness by vibratory stimulation.

Is it possible that
evolution has provided the felines of this world with a natural healing
mechanism for bones and other organs? Researchers at Fauna Communications
believe so.

Being able to
produce frequencies that have been proven to improve healing time, strength and
mobility could explain the purr's natural selection. In the wild when food is
plentiful, the felids are relatively sedentary. They will spend a large portion
of the day and night lounging in trees or on the ground. Consistent exercise is
one of the greatest contributors to bone, (Karlsson et al, 2001), and muscle
(Roth et al, 2000; Tracy et al 1999), and tendon and ligament strength (Simoson
et al, 1995; Tipton et al 1975). If a cats exercise is sporadic it would be
advantageous for them to stimulate bone growth while at rest. As well, following
injury, immediate exercise can rebreak one and re-tear healing muscle and tendon
(Montgomery, 1989). Inactivity decreases the strength of muscles (Tipton et al,
1975). Therefore, having an internal vibrational therapeutic system to stimulate
healing would be advantageous, and would also reduce edema and provide a measure
of pain relief during the healing process.

One might
question...What about other cats that don't purr??? Stay tuned for the
publication.

Unfortunately there is no easy way to test this hypothesis. Strangely, after
speaking with several of the foremost specialists on animal bones, it was
discovered that there has apparently never been a study on any small cat bones,
not serval, caracal, puma, ocelot, or domestic. Only cheetah and tiger bones
have been studied, and tigers do not purr. Cheetahs do purr, but they are one of
the most unique and specialized forms of the felid family. The cheetah's bones
were found to have dense remodeling (growth), which apparently is found in
carnivores and in humans.

Purring-cat
physiology would have to be compared to non-purring cat physiology to test this
theory. The study would have to be entirely non-invasive.

There are
inherent difficulties in discovering whether purring aids in healing, as
purring-cat physiology would have to be compared to non-purring cat physiology.
The dilemma is that most all cats purr, even under duress. They are even capable
of producing a purr following a laryngectomy (Hardie et al, 1981), due to
vibration of the diaphragm (Stogdale and Delack, 1985). A naturally occurring,
non-purring cat is very rare, and this effect is usually associated with a
physical problem. Cats that have physical problems related to purring cannot be
admitted to the study because of the possible variables presented by the
physical disability. Therefore, any research would have to be non-invasive and
observation based.

Given the data on
anabolic frequencies, fracture and healing research, the exact match of the
frequencies and amplitudes of the cat's purrs to vibrational therapy research,
time proven adages, biomechanical therapy, studies on tendon and muscle repair
and Dr. Cook's study, it is certainly not a leap of faith to speculate that the
cat's purr is a healing mechanism. Having a natural way to increase strength,
and decrease healing time, would indeed be very advantageous and would explain
the purr's development.

It is
suggested that purring be stimulated as much as possible when cats are ill or
under duress. If purring is a healing mechanism, it may just help them to
recover faster, and perhaps could even save their life.

We are currently
gathering veterinarian case studies and beginning a study to test the cats'
purr-healing theory. No cats will, or have been harmed in this study. All of
Fauna Communication's studies are non-invasive. We need your help for this
research. We thank you for your support!

****Please note,
before submitting your e-mail to our site we e-mail you and request your
permission. We will not mention your name or e-mail address, unless you
request it. Alsobefore you e-mail us on this topic, we are
not
attempting to disprove the contentment theory, we are merely supplying an
additional hypothesis as to what the purr may do. Additionally, this
research is not designed to determine how the purr is created.

Abstract:
A current hypothesis suggests the purr indicates contentment, however, cats purr
when they are severely injured or frightened. Forty-four felids were recorded
including cheetahs, ocelots, pumas, domestic cats, and servals. A Sony TCD-D8
Digital Audio Recorder (DAT) and Statham Radio microphones recorded the purrs.
FFTs and spectrographs were performed using National Instrument's Polynesia. An
accelerometer was also used to measure domestic cat purrs. Every felid in the
study generated strong frequencies between 25 and 150 Hz. Purr frequencies
correspond to vibrational/electrical frequencies used in treatment for bone
growth/fractures, pain, edema, muscle growth/strain, joint flexibility, dyspnea,
and wounds. Domestic cats, servals, ocelots, and pumas produce fundamental,
dominant, or strong frequencies at exactly 25 Hz and 50 Hz, the two low
frequencies that best promote bone growth/fracture healing [Chen et al., Zhong.
Wai Ke Za Zhi. 32, 217--219 (1994)]. These four species have a strong harmonic
exactly at, or within 2 Hz of 100 Hz, a frequency used therapeutically for pain,
edema, wounds, and dyspnea. An internal healing mechanism would be advantageous,
increasing recovery time and keeping muscles and bone strong when sedentary.
[Published with permission from the New Zealand Veterinary Journal; work
supported by Endevco.]

Rubin CT.
Strain mediated augmentation of bone mass and morphology: Is it
possible to harness the anabolic potential of mechanical
stimuli without necessarily requiring exercise? Presentation at the Wellcome Trust, 1999

Rubin C,
Zhi RH, Hadjrargyrou M. The expression of
a novel and a known gene, unregulated by disuse is down regulated by
anabolic mechanical stimulation. Presentation, American Society of
Bone and Mineral Research, 1999

Wolpert L,
Evans D. The evolutionary psychology of
depression. In: Crisp, Arthur (ed). Every family in the land. Chapter 4,
Self-inflicted, social adaptation or biological destiny?
Models of psychopathology and their relationship to stigmatization.
The Royal Society of Medicine, London; The Royal College of Psychiatry,
London; Robert Mond Memorial Trust, www.stigma.org online book.